Precipitation hardenable, corrosion resistant, chromium-nickel stainless steel alloy



2,861,883 Pa tented Nov. 5, 1958 PRECIPITATION HARDENABLE, CORROSION RESISTANT, CHROMIUM-NICKEL STAINLESS STEEL ALLOY Norman S. Mott, Westfield, N. J., assignor to Cooper Alloy Corporation, Hillside, N. J., a corporation of New Jersey N Drawing. Application February 9, 1956 Serial No. 564,350

8 Claims. (Cl. 75-128) This invention relates to stainless steel alloys, and more particularly to a precipitation hardenable, corrosion resistant, chromium-nickel stainless steel alloy.

A most popular stainless steel alloy is a high chromiumnickel steel, especially the so-called 188 stainless steel. In general, this stainless steel is not hardenable, but was found to be hardenable by the addition of berryllium, which is costly.

The primary object of the present invention is to generally improve stainless steel alloys. A more particular object is to provide a precipitation hardenable stainless steel alloy which does not require the addition of beryllium, and which therefore may be made at lower cost.

The alloy is desirable in situations in which precipitation hardening is wanted, with high strength, and fair ductility. It is good for corrosion-erosion-abrasion resistance. It is also good for stressed parts in corrosive applications.

When handling erosive chemicals .in A151 316 type stainless steel pumps it has been found that the metal erodes and wears away so fast as to make its use impractical. lncreased hardness will prevent this but only if a fair amount of ductility and shock resistance could be-retained would such hardness be tolerated, because surging and pounding will fracture hard brittle metal. If no shock or pounding were present a hard brittle corrosion resisting metal such as Duriron or VZB could be used instead of the more ductile 316 type alloy. In the present alloy the general base type and characteristics of a 316 type alloy are retained, with the additional ability to resist erosion, and there is a resulting combination of higher hardness, with retained ductility.

. The new alloy is based on the addition of molybdenum and silicon. No copper is used, as well as no beryllium. The molybdenum is used in a range of from 3 to 5% and .silicon in arange of 2.5 to 4%.

' Specific examples of my improved alloy, with its resulting hardness and physical properties and corrosion resistance, are given in the following table. The chemical analysis is given in percentage by weight.

Table I Corrosion tests:

65% HNO at boiling .0134 0146 50% His 0 at 80 F 00009 .00007 5% H01 at 80 F 0203 0196 In the foregoing and succeeding tables, BHN means Brinell hardness number; WQ means water quenched; and PH means precipitation hardened after water quenching. Considered more specifically, the heat treat- 5 ment consisted of heating the metal at 2000 F. for one hour, followed by water quenching, after which the water quenched sample was held for eight hours at a temperature of 925 F. and then air cooled or furnace cooled for the precipitation hardening.

In the above table TS refers to the tensile strength of a specimen in pounds per square inch. YSf means the yield strength in pounds per square inch at the customary 0.2% offset, that is, departure from proportionality to show that the elastic limit has been exceeded.

El refers to the elongation of the specimenin percentage, before rupture. RA refers to the reduction 'of area in percentage at the time of rupture.

The decimal figures given in the above table for the corrosion tests are -I. P. M. figures, meaning the corrosion rate in inches of penetration per month when immersed in the indicated acid.

Alloys X-lS and XX-l8 in the above table illustrate hardening by the addition of high molybdenum and high silicon, Without beryllium and copper.

Further examples of the invention are as follows:

Table I-A 'IestNo X4813 X4800 X-isF O perceut 052 071 042 or--- d 20.05 20.00 20.80

Ni- 9. 05 3. 9s 8. .72 .70 .70 4.18 4.11 4.12 3.68 3.32 3. 82

The fact that both molybdenum and silicon must be used, and that either will not alone serve, is demonstrated by the following table:

Table II X-l X47 X-18AA X-18B Alloys X-1 and X-18AA show that when the silicon is very low a molybdenum content of appreciable high percentage will not alone induce precipitation hardening in essentially austenitic chromium-nickel alloys. Alloy X-47 shows that a high silicon percentage will not alone induce precipitation hardening in this type of alloy. Alloy X-18B shows that high molybdenum along with high silicon induces precipitation hardening by .well over 50 points of Brinell hardness. This value of 50 is significant for many purposes because it has been found that a difference in hardness of at least 50 points is needed between metals which are in sliding contact with one another to prevent galling or seizing. Thus a. journal and its bearing, if made of stainless steel, should have a difference in hardness of at least 50 BHN, andsuch a difference has been produced by precipitation hardening of alloy X-18B in Table II. The hardening is accompanied also by an increase in tensile strength. In Tables :I.and-I'A the gain is much more than 50. V

I The effect of increasing the molybdenum content is explored in the following table:

Table III X-30 X-18 X-31 X-32 O percent .040 033 .038 V 052 1012;. 20. 15 20. 25 19. 90 19. 95 .NL- 8. 90 8. 90 8. 90 8. 95 M11 87 86 .87 .84 M0. 2. 97 '3. 96 4. 92 6. 50 SL. 3. 50 3. 46 3. 38 3. 62 BH VY-Q v .241 262 285 321 in alloy X-30 with a molybdenum content of 2.97% .the hardness was increased 70 points by precipitation hardening to 311; in alloy X18 with the molybdenum content increased to 3.96% the precipitation hardening rose 79 points to 341; and in alloy X-3l with the molybdenum content increased to 4.92% the precipitation hardening increased 90 points to 375. However, in alloy X3 2 with 'the molybdenum content greatly increased to 6.50% the precipitation hardening rose only 67 points to 388. The gains in hardness, (PH) number were 30, 34, and 13, thus showing that'from either viewpoint a -molybdenum'conten't of over say 5% produces very little increase in precipitation hardening effect. It will be noted that in'all of"thes'e"alloys'the silicon content was high and substantially constant, and that the alloys are all without copper.

Alloys requiring large quantities of molybdenum, 'say :more than 5%, would probably be undesirable comrnercially because of high cost. Thus the X32 alloy in {Table III would be undesirable for that consideration alone. Of course, while molybdenum is expensive, it is only moderate in cost compared to beryllium, which is not .required'at all in any of the alloys disclosed in this specification.

The effect of increasing the silicon content is shown in the following table:

'In alloy X-37 the s'ilicon' was 1.68% and after precipitation hardening thehardnessincreased 56 points to 285. In alloy X-38 the silicon was raised to 2.62% :and the hardness increased 70 points to 311. In alloy .X- l8 the silicon was raised to 3.46% and precipitation hardening increased the hardness 79 points to 341. In 'alloy iX-39 the silicon was increased to 4.52% and there was -a resulting gain in hardness but it was found that :the specimen cracked badly, thus showing that a practical .limit for the silicon content is about 4%. It will be :noted' that inall of these alloys the molybdenum content was substantially constant and that no copper was used.

The effect of increasing the chromium content is con- Isidcred in'the following table:

' 4 Table V Alloys X-40 and X-44 are without copper, but have a high molybdenum and silicon content. In .alloy X-44 the chromium has been increased from about 18 to 22%, with a resulting increase in precipitation hardness of 34 points. This shows that increasing the chromium percentage has some influence in increasing the precipitation hardening effect inalloys whicharefree'of copper.

In Table V it will 'beselen that I' increased chromium from 18 to over 22%. It'wasunnecessary'to'carrythis increase further because it is known in "this workithat increased chromium ordinarily increases hardness,'and the experiments in Table V were sufiicient'to'show that this general property applies also to the'jpresent alloys, without copper. It is also known in this art thatitis not practical to carry the increase'of chromium above 30%, and it is for that reason'that I'co'nsider the maximum range of chromium to extend'up I to 30%. Increasing chromium increases hardness and therefore reduces the requirements on otherlelements, but optimum general properties are not necessarily "obtained, and I therefore consider a chromium'range up to 25% to be a more preferable range.

The elfect of increasing the amount of nickel is considered in the following table:

these molybdenum, silicon alloys reduces the precipitation hardening effect.

In all of the foregoing analyses, the'iron "contentis not included, but it -is, of course, understood that the balance is iron, subject to the presence of small amounts of impurities incidental to the usual melting practices when dealing with ferrous metals. To cover this situation I may state that in addition to the elements named in the analyses, the remainder is substantially all iron.

The maximum carbon content should be no higher than, say, 0.08%. M

It will be understood that the alloys are fullyresistant to salt spray. Indeed, the'acid tests shown in Table I are much more severe than a saltspray test.

The alloys are weldable by using welding'rodsc'ifthe same general composition as the alloy being welded.

The results of the foregoing tables of tests'i'n'ay be summarized withthe percentages rounded ofi as follows. The desired result may be obtained by the addition of molybdenum in a range of from 3 to 5 and silicon in a range of 2.5 to 4%. Increasing the amount ofchroinium produces an increase in hardnss and thereve rse asensss is true for nickel, that is, an increase in nickel decreases hardness. In general, when dealing with the alloys described in this specification, the yield and tensile strengths increase with the hardness, although not necessarily in linear proportion.

The present application is a continuation-in-part, and largely a division of my copending parent application Serial No. 490,698, filed February 25, 1955, now abandoned, and entitled Precipitation Hardenable, Corrosion Resistant, Chromium Nickel Stainless Steel Alloy. That application disclosed what is referred to commercially by the assignee of these applications as its PH-55 series of alloys, divisible into three alloys designated as PH-55A, PH-55B, and PH-55C."

The PH-SSA alloy is characterized by high strength and high hardness with fair ductility, and is intended for erosion and abrasion resistance or for stressed parts in corrosive applications. It is an alloy which answers an objective decided upon by the stainless steel casting industry, expressing the need for a comparatively hard stainless steel alloy having a fair amount of ductility for corrosion-erosion resistance with a minimum of 5% elongation, at 350 BHN hardness. The corrosion resistance was desired to be about equal to that of a CF-SM alloy of the Alloy Casting Institute or ACI (which alloy has 0.08% carbon maximum, 18% chromium, 8% nickel, and 3% molybdenum).

The PH-55B alloy is a ductile alloy characterized by high strength and medium hardness, and intended for shock resistance and high stresses in corrosive applications. It answers an industry-decided objective expressing the need for a high strength ductile stainless steel alloy for structural purposes, with a tensile strength as near as possible to twice that of a cast alloy of the ACI known as CF8 (which has 0.08% maximum carbon, 18% chromium, and 8% nickel, and a tensile strength of 77,000 p. s. .i. average). In this industry objective the yield strength was to be over 100,000 p. s. i., the elongation was to be over 10%, and the corrosion resistance was to be as good as that of the CF-8. High hardness was not too important.

The PH-55C alloy is characterized by very high hardness and low ductility for use in non-stressed, corrosion resisting parts. It answers an industry-decided objective expressing the need for an extremely hard stainless steel alloy having high abrasion resistance and not requiring ductility.

The present application is directed to the PH-SSA alloy. The PH-55B and PH-55C alloys are disclosed in companion applications which, like the present application, have been divided from my aforesaid parent application Serial No. 490,698, but which technically may be considered to be continuations-in-part rather than true divisional applications, because of added examples.

Considered as a high molybdenum, high silicon alloy, without copper or beryllium, a broad range of composition which will effectuate the invention is as follows:

Percent C Under .08 Cr 18-25 Ni 8-12 Mo 3-5 Si 2.5-4

The preferred range is narrower than the broad range given above, and a preferred or narrowed range may be given as follows:

It is believed that the composition and behavior ofmy improved hardenable stainless steel alloys, as well as the advantages thereof, will be apparent from the foregoing detailed description. The new alloys are low in cost and high in corrosion resistance. They are soft enough in the quench annealed condition to be readily machinable and may be precipitation hardened by a comparatively low emperature heat treatment. The increase in hardness makes the alloys resistant to galling and erosion. The alloys are resistant to salt spray and acids. The alloys have the high chromium and nickel content of a regular 18-8 type of stainless steel, and therefore retain the advantages of that type of stainless steel.

It will be apparent that while I have set forth specific examples of my improved alloys, changes may be made without departing from the scope of the invention, as sought to be defined in the following claims.

I claim:

1. A low cost precipitation hardenable alloy of high corrosion, erosion, abrasion resistance and fair ductility adequate to be used for stressed parts, said alloy consisting essentially of a chromium nickel stainless steel of the type known generally as 18 and 8, having added thereto molybdenum and silicon, the molybdenum ranging from 3% to 5%, and the silicon ranging from 2.5% to 4%, said alloy having a carbon content not exceeding 0.08% and said alloy being free of beryllium and copper.

2. A low cost precipitation hardenable alloy of high corrosion, erosion, abrasion resistance and ductility adequate to be used for stressed parts, said alloy being of the general type known as 18 and 8 stainless steel, said alloy having a range of from 18% to 25% chromium and -8% to 12% nickel, said alloy having added thereto molybdenum in a range of from 3% to 5%, and silicon in a range of from 2.5% to 4%, the remainder being essentially iron with a carbon content not exceeding about 0.08% and said alloy being free of beryllium and copper.

3. A low cost precipitation hardenable alloy of high corrosion, erosion, abrasion resistance and ductility adequate to be used for stressed parts, saidl alloy being of the general type known as 18 and 8 stainless steel, said alloy having a range of from 19.5% to 20% chromium and 9% to 10% nickel, said alloy having added thereto molybdenum in a range of from 3.75% to 4.25%, and silicon in a range of from 3.25% to 3.75%, the remainder being essentially iron with a carbon content not exceeding about 0.08% and said alloy being free of beryllium and copper.

4. A low cost precipitation hardenable alloy having approximately the following chemical analysis: carbon 0.033%; chromium 20.25%; nickel 8.90%; manganese 0.86%; molybdenum 3.96%; silicon 3.46%; and the balance of the .alloy being substantially all iron, said alloy being free of beryllium and copper, the said alloy when hardened being characterized by good corrosion, erosion, abrasion resistance and fair ductility adequate for stressed parts.

5. A low cost precipitation hardenable alloy having approximately the following chemical analysis: carbon 0.052%; chromium 19.82%; nickel 9.05%; manganese 0.75%; molybdenum 4.18%; silicon 3.52%; and the balance of the alloy being substantially all iron, said alloy being free of beryllium and copper, the said alloy when hardened being characterized by good corrosion, erosion, abrasion resistance and fair ductility adequate for stressed parts.

6. A low cost precipitation hardenable alloy having approximately the following chemical analysis: carbon 0.071%; chromium 20%; nickel 8.96%; manganese 0.70%; molybdenum 4.11%; silicon 3.82%; and the balance of the alloy being substantially all iron, said alloy being free of beryllium and copper, the said alloy when hardened being characterized by good corrosion, erosion, abrasion resistance and fair ductility adequate for stressed parts.

when hardened being characterized by good corrosion, erosion, abrasion resistance and fair ductility adequate for stressed parts.

References Cited in the file of this :patent UNITED STATES PATENTS 2,051,415 'Payson Aug. 18, 19316 FOREIGN PATENTS 53,768 Netherlands Jan. 15, "1943 OTHER REFERENCES Metals Handbook, 1954 Supplement, pages. 34-41. Published by the American Society for Metals, Cleve- :alloy being free of beryllium and copper, the said alloy 15 land, Ohio. 

1. A LOW COST PRECIPITATION HARDENABLE ALLOY OF HIGH CORROSION, EROSION, ABRASION RESISTANCE AND FAIR DUCTILITY ADEQUATE TO BE USED FOR STRESSED PARTS, SAID ALLOY CONSISTING ESSENTIALLY OF A CHROMIUM NICKEL STAINLESS STEEL OF THE TYPE KNOWN GENERALLY AS 18 AND 8, HAVING ADDED THERETO MOLYBDENUM AND SILICON, THE MOLYBDENUM RANGING FROM 3% TO 5%, AND THE SILICON RANGING FROM 2.5% TO 4%, SAID ALLOY HAVING A CARBON CONTENT NOT EXCEEDING 0.08% AND SAID ALLOY BEING FREE OF BERYLLIUM AND COPPER. 